Optical signal to noise ratio (OSNR) is a key performance parameter in wavelength division multiplex (WDM) optical networks to predict the bit error ratio (BER) of the system. It is the ratio of the useful signal to noise. Prior art OSNR measurements and calibration have made use of the interpolation method.
In the interpolation method, OSNR for a given optical channel is obtained by measuring a total signal power and a noise power in a passband associated with the optical channel. Typically, the noise is dominated by amplifier spontaneous emission (ASE) noise generated within optical amplifiers in the optical network. ASE may be measured in spectral gaps either side of the optical channel, normalized to 0.1 nm bandwidth. This is called the linear interpolation method because the noise power is averaged from the values of ASE noise present to the left and to the right of the optical channel.
FIG. 1 shows a representative prior art WDM spectrum comprising three 10 Gb/s optical channels. The three signal envelopes appear as optical power peaks 102a-102c on a noise background level 101. The passband OSNR 103 is usually estimated from a ratio of an optical power reading Psignal of power peak 102b at the center wavelength λi and an average of noise power measurements Pnoise—L 104left and Pnoise—R 104right taken either side of the optical channel of interest 102b. 
Thus the OSNR can be calculated as:
  OSNR  =            P      signal                                P          Noise_L                +                  P          Noise_R                    2      
For WDM optical networks and all-optical networks (AON), which do not have any optical filters in their optical path, the interpolation method gives an accurate OSNR reading.
However, many networks do include one or several optical filters, such as channel filters and multiplexers, with consequent distortion of the noise spectrum. In AON networks in-line optical filters built into reconfigurable optical add-drop multiplexers (ROADMs) or dispersion compensation fiber Bragg gratings (FBGs) will also suppress the noise in between optical channels.
In dense WDM (DWDM) systems operating at high data rates with modulation formats like RZ, CRZ, CSRZ and similar, the modulation bandwidth can be so high that the modulation bands overlap the spectral gaps between the optical signal channels. Under such conditions the ASE noise power is not directly accessible in the inter-channel spectral gap.
The measurement of the noise power in the inter-channel spectral gaps, used by the OSNR interpolation method, will not give an accurate indication of the noise present at the channel wavelength. Under such conditions the interpolation method is no longer reliable for producing accurate measurements.
FIG. 2 shows a representative prior art WDM spectrum comprising two optical channels modulated at 10 Gb/s with the two signal envelopes appearing as optical power peaks 202a and 202b on a noise background level 201. Using the interpolation method, an interpolated OSNR 204 is obtained. However, the actual noise level within the passband is given by curve 205b so that the actual in-band OSNR 203 is substantially smaller than the interpolated OSNR 204. The reason is that the noise spectrum e.g. of ASE noise is no longer flat, but is distorted by cumulative filter characteristics 206a, 206b of the in-line filters mentioned above.
There are a number of alternative ways for accurately determining OSNR in systems such as those mentioned above.
The time resolved optical gating (TROG) method involves signal deactivation. The channel signal is switched off or blanked for a duration sufficiently short in order not to permit gain levels of optical amplifiers in the WDM optical network to be affected. During the blanking, the in-band noise level is measured. The OSNR is then derived from the in-band noise level and the power in the channel with the signal present. There are major drawbacks with this method. It cannot be performed in a live system without service interruption, which makes it unsuitable for routine monitoring.
In addition, blanking the channel signal can cause instability of ASE noise, as automatic gain control in the optical amplifiers may change the ASE noise level when the signal is switched off. The noise power level reading may be rendered inaccurate in this situation, yielding an inaccurate OSNR value.
Another method is based on the recognition that the noise, principally ASE noise, has a random polarization, whereas useful signals have a definite polarization. Thus, by determining the polarization of a particular signal in an optical channel, optical power measurement at the orthogonal polarization may be used to estimate the in-band noise level PNoise—in-band. The noise level can then be subtracted from the combined signal and noise power measured at the useful signal polarization to obtain the signal power Psignal. From the two resulting values, the OSNR is calculated from the equation:
  OSNR  =            P      signal              P              Noise_in        ⁢                  -                ⁢        band            
In effect, the optical signal is suppressed to permit the noise power at the signal-wavelength to be measured. This in-band OSNR testing principle is sometimes referred to as polarization controlling or nulling.
Prior art OSNR monitoring apparatus making use of this principle has been disclosed by Chung (US Patent Application 20040114923), as shown in FIG. 3.
An optical WDM signal is introduced into a polarization controller 22, from where it is passed through a tunable optical bandpass filter 24 and split into two components by a polarization separator 42 along paths Path3 and Path4. Each component is converted to an electric signal by photodetectors 30a and 30b whose output is converted into digital form by analog-digital converters 32a and 32b respectively, which feed into a power calculator 34 followed by an OSNR calculator 36.
For each wavelength setting of the tunable optical bandpass filter 24 a corresponding polarization state of a signal can be determined by varying the state of the polarization controller 22, thereby permitting appropriate polarization splitting to be accomplished.
The apparatus of Chung presents certain disadvantages. Since it is difficult to make a tunable filter with high dynamic range that has a single mode fiber (SMF) output and narrow bandwidth, in practice SMF pigtailed elements such as polarization controller 22, polarization separator 42 or photodiodes 30a, 30b cannot be readily coupled to the output of tunable optical bandpass filter 24 without incurring considerable insertion loss.